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Re: Dinosaurs and birds
On Wednesday, April 4, 2007, at 05:41 PM, firstname.lastname@example.org
And David M. asked about bats too: the problem with phrasing the
question like this is that birds have many of their own adaptations to
flight, and just because bats or pterosaurs have their own derived
solution to a problem doesn't mean that a proto-bird that lacks the
derived avian condition is magically exempted from a biomechanical
Excellent point. However, on the flip side, the derived avian
condition can largely hide what the minimum flight conditions are. For
example, a common error is to assume that chickens represent a good
minimum flight model, or that pigeons with cut supracoracoideus tendons
demonstrate the basal pre-supracora. condition. The derived condition
has become reliant on a supracoracoideus pulley; that does not mean
that avian flight required one primitively. The pulley has distinct
advantages, of course, but it may not have been biomechanically
As noted by Baier et al (Nature, 2007), the theropod glenoid capsule
is shaped such that it is open on two ends (rather than being a true
ball and socket joint). It's orientation in all maniraptorans
(including avialans) is such that it cannot prevent dislocation of the
humerus during the flight stroke unless their is tendonal and muscular
opposition to the movement.
True. Though David Baier also has found that the supracoracoideus
pulley does not stabilize the shoulder joint (pers. comm). Rather, the
supracoracoideus destabilizes it further. The Acromocoracohumeral
ligament is the primary mechanism for joint fidelity in derived avians.
It may or may not have been in basal birds.
Niether of these derived solution that are unique to each group
implies that a stem-group bird or early avialan that lacks the _avian_
derived condition can be exempted from these biomechanical problems.
True, but the fact that a given solution is important in derived avians
doesn't mean that the basal condition required the same features, as
well. It's a tough problem to crack. Like you said, the lack of given
features in separate flying lineages is not particularly informative,
but the presence in derived groups isn't always informative, either.
In other words, a "pulley-like supracoracoideus" is only necessary for
flight if you lack other derived adaptations to solve the
The supracoracoideus pulley system is only necessary if a rapid
upstroke is important. Since the supracoracoideus destabilizes the
joint (along with pec. major), it falls almost entirely to ligamentous
features to provide stability in modern birds. The supracoracoideus
probably doesn't have much to do with stability of the shoulder in
derived birds. You may have some additional data suggesting that it
was more stabilizing primitively, though. I'm mostly just going on my
chat with David Baier, since he's looked at the joint fidelity more
than I have (though his result in that regard made sense to me).